R. Kovacevic

Principles of abrasive water jet machining

Preface.- Nomenclature.- Introduction.- Classification and Characterization of Abrasive Materials.- Generation of Abrasive Water Jets.- Structure and Hydrodynamics of Abrasive Water Jets.- Material-Removal Mechanisms in Abrasive Water-Jet Machining.- Modeling of Abrasive Water Jet Cutting Processes.- Process Parameter Optimization.- Geometry, Topography and Integrity of Abrasive Water-Jet Machined Parts.- Alternative Machining Operations With Abrasive Water Jet.- Control and Supervision of Abrasive Water-Jet Machining Processes.- References.

State of the Art of Research and Development in Abrasive Waterjet Machining

Thermodynamic analysis of material removal mechanisms indicates that an ideal tool for shaping of materials is a high energy beam, having infinitely small cross-section, precisely controlled depth, and direction of penetration, and does not cause any detrimental effects on the generated surface. The production of the beam should be relatively inexpensive and environmentally sound while the material removal rate should be reasonably high for the process to be viable. A narrow stream of high energy water mixed with abrasive particles comes close to meeting these requirements because abrasive waterjet machining has become one of the leading manufacturing technologies in a relatively short period of time. This paper gives an overview of the basic research and development activities in the area of abrasive waterjet machining in the 1990s in the United States.

Surface texture in abrasive waterjet cutting

Abstract When cutting thick material with an abrasive waterjet, the surface area can be characterized by two texture types. At the top of the cut, the surface texture is smooth. At the bottom, the slurry jet exiting the workpiece forms large striations in the surface. An investigation was conducted to experimentally determine the influence of the abrasive waterjet cutting parameters on surface texture. A mathematical model was developed characterizing the correlation between the surface roughness and the abrasive waterjet cutting variables.

Monitoring the depth of abrasive waterjet penetration

Abstract In order to control the uniformity of the abrasive waterjet penetration into the workpiece, it is necessary to devise a monitoring methodology that can indirectly monitor the depth of abrasive waterjet penetration. It was shown that the workpiece normal force generated by an abrasive waterjet could be used as the indicator of the depth of jet penetration, and that a force-feedback control holds promise as an effective way to regulate the depth of jet penetration. The effects of different abrasive waterjet process variables on both the depth of cut and the workpiece normal force are discussed.

On-line analysis of hydro-abrasive erosion of pre-cracked materials by acoustic emission

Acoustic Emission (AE) sensing technique is used as a tool for on-line monitoring of hydro-abrasive erosion (HAE) of pre-cracked multiphase materials. As reference materials, five types of concrete materials were used for the experimental study. Compression tests were performed to determine the mechanical properties and the failure behavior of these materials. Erosion parameters, such as abrasive particle velocity, local exposure time, and abrasive mass flow rate were varied during the experiments and AE-signals were acquired. The trends exhibited by the time domain and frequency domain AE-signals with change in process parameters and material properties were analyzed. The results indicate that acoustic emission signal is capable of revealing the diAerent material removal mechanisms occurring in pre-cracked multiphase materials when subjected to hydro-abrasive erosion. Visualization studies performed on the erosion site provide more insight into the physics of the process and verify the observations made from the AE-signals. Finally, it is concluded that due to its capability to quantify the amount of material removed, AERMS could be considered as a parameter for monitoring the material removal process. ” 1999 Elsevier Science Ltd. All rights reserved.

Modeling of the influence of the abrasive waterjet cutting parameters on the depth of cut based on fuzzy rules

Abstract Currently, the abrasive waterjet cutting parameters for the milling operation have to be determined by a combination of prior experience and trial and error. It is shown that the selection of the abrasive waterjet cutting parameters for a required depth of cut in the given material can be effectively done by applying the principles of the fuzzy set theory. This approach will eliminate the need for extensive experimental work in order to select the magnitudes of the most influential abrasive waterjet parameters on the depth of cut. Fuzzy logic provides a methodology and imitation of a humans way of making decisions which is very useful in such applications where the mathematical model of the process does not exist, and one of such processes is indeed abrasive waterjet cutting. A number of case studies are performed to verify the validity of the proposed methodology for selecting the abrasive waterjet cutting parameters in order to achieve the predetermined depth of cut.

Fundamental investigations on concrete wear by high velocity water flow

Abstract A primary concern in the design of marine constructions is the wear and erosion of concrete structures by high velocity water flow. The mechanism of concrete failure for this type of loading is not well understood. By using high velocity water jets to simulate the loading, the authors observed the general behaviour of concrete during failure and investigated the influence of water velocity and exposure time. The results show that the interface between hardened cement paste and aggregate grains plays the main role in the fracture process. It is found that a critical threshold velocity wc and a critical threshold exposure time tc must be achieved in order to induce the erosion process. A mathematical relation between both parameters and the erosion intensity is found through mass removal measurements.

Cutting Force Dynamics as a Tool for Surface Profile Monitoring in AWJ

Abrasive waterjet cut surface is characterized using static and dynamic characterization techniques. A novel method of auto regressive moving average model identification called model distance method is utilized here for surface profile and dynamic force characterization. More information about the surface profile generating mechanism is derived through wavelength decomposition of the ARMA models. The dynamic workpiece normal force in abrasive waterjet is influenced by process parameters such as fluctuations in water pressure, change in abrasive flow rate, vibration of the positioning system, traverse speed, nozzle diameter, etc. An attempt has been made in this paper to link the dynamics of the process to the quality of the generated surface. The feasibility of using the dynamic workpiece normal force as a parameter for on-line monitoring of the surface profile generated by abrasive waterjet is also investigated.

Statistical character of the failure of multiphase materials due to high pressure water jet impingement

In this study, fracture experiments on multiphase material samples have been carried out using high speed water jets. Based on fracture geometry measurements and on grain analyses it was found that the fracture of this type of materials by water jet impingement is a highly localized process at low pressure ranges. Beyond a critical pressure range of about 30 times the materials tensile strength a change in the material behaviour was observed. This result is in agreement with a theory suggested by Powell and Simpson. To explain the local character of the failure process, a simplified fracture model is introduced which resulted in a relation between a fracture probability parameter and the fracture width in the damaged materials.

An energy balance of high-speed abrasive water jet erosion

Abstract High-speed abrasive water jetting is an alternative tool for machining engineering materials. The energy dissipation processes involved in this erosion process have not been investigated systematically. In this paper, a model is developed to calculate the energy dissipation in workpieces eroded by abrasive water jets. By introducing an energy dissipation function χ(Φ). the model enables the estimation of the energy absorption as a function of the erosion depth. The energy dissipation function can be expressed by a second-order polynomial approximation. Measurements of the reaction forces on the exiting slurry after the erosion process, material removal experiments and fracture tests are conducted to separate the components of the energy dissipation parameter, such as damping, friction and erosion debris generation.